This invention relates to techniques for aligning microscopic patterns, and more particularly, to techniques for aligning a photolithographic mask with a semiconductor wafer pattern, as is required in semiconductor integrated circuit fabrication.
In the fabrication of semiconductor devices by photolithographic techniques, a semiconductor wafer is coated with a photosensitive film referred to as photoresist and selectively exposed to light projected through a mask in close proximity to, or in contact with, the wafer coating. Development of a selectively exposed photoresist, followed by an etching and diffusion into the wafer, permits patterns of various conductivity or different materials to be defined on the wafer surface. The complete fabrication of modern integrated circuits requires several such printing steps to be performed successively with each mask exposure being in precisely controlled registration with previously defined wafer patterns. Because the patterns on the masks and wafer have extremely small dimensions, a microscope is normally required for making the mask alignment.
To give sufficient resolution to meet stringent accuracy requirements, the microscope must have a large numerical aperture, and as a consequence, it has a small depth of field. However, to permit the operator to move the wafer relative to the mask to achieve registration without abrading the photoresist coating, the mask and wafer must be physically separated. This separation, which may typically be on the order of 20-60 microns, may be greater than the depth of field of the high-power microscope needed for making the registration.
Since the operator cannot observe the mask and wafer simultaneously, it is typically necessary for him to make separate alignment of the separated mask and wafer, then again observe the alignment after bringing them into contact, and, if necessary, separate them again for realignment. This process of separating and contacting, which is repeated until the desired alignment accuracy is achieved, is time consuming and frequently results in abrasion of the photoresist, as well as degradation of the mask and wafer features.